Address PR review: backup correctness/security + drop hardening

Fixes from the code review of this branch:

- Backup requires a running container (it runs via `docker exec`, which
  can't run on a stopped one). Removed the misleading "Backup" button
  from the stopped-project actions, added an explicit running check with
  a clear error, and corrected the doc comment. (H1)
- jq sanitization fallback no longer leaks secrets: if ~/.claude.json
  can't be parsed, the backup substitutes an empty object and warns to
  stderr instead of copying the raw file (which held primaryApiKey /
  oauthAccount). Verified the raw key never reaches the archive. (H2)
- Dropped-file paths typed into the terminal are now always single-quoted
  (with '\'' escaping), not only when they contain whitespace — a name
  like `foo$(whoami).txt` was previously sent raw into the shell. (M2)
- write_bedrock_static_credentials checks the exec exit code via the new
  exec_oneshot_env_status and fails loudly on a write/chmod error instead
  of silently reporting success. exec_oneshot keeps its
  ignore-exit-code behavior so list_container_files is unaffected. (M4)
- Backup removes a partial/truncated archive on any stream error and
  treats a non-zero tar exit code as failure (a truncated gzip was
  previously reported as success). (L1)
- Dropped files are capped at 256 MiB to avoid ballooning host RAM
  (the file is read fully into memory then re-tarred). (M3)
- Stopped excluding .git/objects from the backup so git history,
  including unpushed commits, is preserved faithfully. (L3)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/components/projects/ProjectCard.tsx

@@ -511,11 +511,6 @@ export default function ProjectCard({ project }: Props) {
             {isStopped ? (
               <>
                 <ActionButton onClick={handleStart} disabled={loading} label="Start" />
-                <ActionButton
-                  onClick={handleBackup}
-                  disabled={loading || backingUp || !project.container_id}
-                  label={backingUp ? "Backing up…" : "Backup"}
-                />
                 <ActionButton
                   onClick={async () => {
                     setLoading(true);