fix(app): address the step 5 review and emulator findings

- A store a cancelled attempt already made for the chosen key (its
  .gpg-id names exactly that key) no longer counts as occupied:
  choosing the key again commits and moves on instead of refusing.
- SSH key import reads at most 100 kB whatever the provider reports,
  like the PGP import, instead of reading the stream whole.
- PGPKeyManager replaces a key through a temporary file moved over it
  atomically, so a failed write keeps the old key; listings only read
  .key files, so a stray temporary file is never taken for a key.
- "View generated public SSH key" goes through the settings result
  launcher, so the preference visibility refreshes afterwards.
- A plain start from LaunchActivity replaces the task: an open
  MainActivity (e.g. one whose store vanished) is no longer left
  below the new one, and one back leaves the app.
- The PGP keys screen's import FAB carries its content description
  itself; M3 cleared the icon's.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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