Close what two reviews found in the Files tab transfers

Two independent reviews of 2c9482a, one for correctness and one against the
threat model. Between them they found two ways to lose a file, one way for a
container to choose a Windows save destination, and a rule that made every
dotfile unsavable. Every finding below was demonstrated against a real
container before being fixed, and the fixes are demonstrated the same way.

## The download was accepting truncated reads and refusing whole ones

The `[ -f ]` bracket around the read was wrong in both directions.

It missed the case that loses data. Truncation *in place* — `> file`, log
rotation, `tar -x`, most build tools — leaves a regular file behind, so `dd`
stopped at the new EOF and exited 0. Measured: a 600 MB source truncated
mid-read delivered 34 MB, which was then renamed over the user's own earlier
copy and toasted as `Saved (34.1 MB)`. Truncate-to-zero did the same and needs
no adversary at all.

And it failed *good* downloads. `dd` already holds the fd, and neither `rm` nor
`mv` can touch an open one — the bytes are complete. But `rm` makes `[ -f ]`
false, so a finished 600 MB transfer of a file a bundler happened to unlink was
deleted, reporting "nothing was saved". The comment claiming the bracket caught
a "mix of two files" was simply wrong; an open fd cannot be a mix.

So the script now measures the file before reading it and puts the answer on
stderr, and Rust checks that at least that many bytes arrived. One rule,
subsuming everything the bracket was for. Verified against a real container:
truncation mid-read and the FIFO race are refused; deletion, rename-over, a
growing file, an empty file and an untouched 600 MB read all pass.

## On Windows the container, not the user, was naming the save destination

`suggested_save_name` split on `/` only, and it feeds the save dialog's
pre-filled name. Backslash is a legal Linux filename character and
`validate_container_path` has no reason to object — `..\..\Users\…` is one
POSIX segment. The Windows common file dialog parses its name box as a path on
Save, so a container-created file called
`..\..\..\Users\vic\AppData\Roaming\Microsoft\Word\STARTUP\x.dotm` put its own
bytes in an auto-loading Office directory on one un-read click. `resolve_host_path`
did not stop it: Word's `STARTUP` and Excel's `XLSTART` are not in the autorun
denylist, which this file's own docs already concede is "losing by
construction" and was never meant to be the boundary here.

The name is now sanitized of every separator, the drive colon and the rest of
what NTFS refuses, so it cannot be a path on any platform this ships to.

## No dotfile could be saved, and the app pre-filled the name that guaranteed it

The write policy judged the leaf for hiddenness, so `/workspace/.env` was
refused *after* the modal and the overwrite prompt — quoting the name the app
itself had suggested. `.gitignore`, `.dockerignore`, `.eslintrc.json`, `.nvmrc`:
all unsavable, while uploading them worked, so a dotfile could go in and never
come out.

`HostPathUse` gains a third mode. `WriteChosenName` drops the leaf check and
keeps every directory rule, and only `download_container_file` uses it — the
dialog is a real boundary for that caller and only that caller.
`download_container_backup` still takes its path over IPC as a string and keeps
the strict rule.

## Smaller, all found by the reviews

  * A download had no ceiling. `dd` resolves through the container's `PATH`,
    which its agent owns with passwordless sudo; a replacement writing forever
    was measured at ~6 GB/s, so one click on a file listed as 2 KB filled the
    host disk with no progress shown and no cancel. Bounded now by what the
    file measured, with slack that is absolute for small files and
    proportional for large ones, so an honest growing log is unaffected.
  * "Framed rather than verbatim" did not stop container text reaching the
    toast headline: `readableRefusal` matches with `includes`, and it has to,
    because the app's own refusals carry those markers mid-sentence. Anchoring
    would break them. The fix is at the injection point — container text is
    clipped to one 200-character line with control characters stripped — plus a
    `max-h-40` on the toast message, which the `detail` block always had and
    this half did not. 8 KB of prose in a `z-[60]` card pushed its own dismiss
    button off-screen.
  * `savingPath` was a scalar while the design deliberately allows concurrent
    saves. Starting a second freed the first's row mid-transfer, and whichever
    finished first cleared both; dismissing the second dialog was enough. It is
    a `Set` now.
  * `setUploading(false)` fired when the command settled, not when the refresh
    finished, so a second click landed mid-relisting.
  * `upload_files_to_container` checked the container directory before the
    picker and then used the unresolved path. A modal has no time limit. It
    re-checks after, which is what `docker::exec`'s doc comment already claimed.
  * `wait_for_exec_exit` flattened a missing exit code to `Some(0)`, which made
    the new `!= Some(0)` check unreachable by construction.
  * `normalize_host_path` did not collapse repeated separators, so the lexical
    system-root rule was silently absent for `C:\\Windows\…`. Not exploitable —
    the resolved pass catches it — but a documented layer that does nothing is
    a trap for the next caller.
  * README still carried the "no host path crosses IPC in either direction"
    claim the previous commit narrowed everywhere else, and HOW-TO-USE
    described the hidden rule without saying it applies to folders only.

480 Rust tests, 602 frontend, no new clippy warnings. Eleven mutations against
the new tests, all killed — two of the first round survived and were rewritten:
one because `split_whitespace` already handled the case I thought I was
testing, one because I had deleted a comment rather than the behaviour.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LHL9ty7arp8FHwvE77ne7y

HOW-TO-USE.md

@@ -1212,13 +1212,17 @@ cannot name a place on your machine — it can only ask for a dialog — and not
 until you pick somewhere in it. Closing a dialog without choosing is not an error: nothing happens,
 and nothing is said about it.
 
-Every one of these routes refuses a location whose path passes through a hidden folder — anything
+Every one of these routes refuses a location whose path passes through a hidden *folder* — anything
 with a component beginning with `.`, such as `~/.ssh`, `~/.cache` or `~/.local/share` — or a system
 location, and it checks both the path as written and where it points after any symbolic links. That
 rule catches more than it strictly needs to, so now and then it will refuse a place you genuinely
 meant, `~/.config` among them. The refusal is a plain sentence saying so; choose a visible location
 such as `~/Documents` or `~/Downloads`.
 
+The *file's own name* is a different matter, and dotfiles are fine: `.env`, `.gitignore` and the
+rest save normally, since you chose the name in the save dialog yourself. Only the folders on the
+way are judged.
+
 If you already keep the project in a folder mounted into the container, the simplest answer is
 usually none of the above: edit the file on your host and it is already inside.