* fix(dictation): preserve composer text when server STT commits

Dictation via the server STT path (MediaRecorder → POST /api/transcribe)
replaced any text already in the composer with the recognized transcript.
Browser SpeechRecognition worked correctly; only the async path was broken.

Root cause: the MediaRecorder.onstop handler calls _setRecording(false)
before awaiting _transcribeBlob's fetch. _setRecording(false) clears the
module-level _prefix. By the time the fetch resolves and _commitTranscript
runs, _prefix is '' — so the existing append logic collapses to
'just the transcript' and overwrites the textarea.

Browser SR dodged this because sr.onend builds the merged string into a
local variable before invoking _setRecording(false). Server STT is async
and could not use that pattern without help.

Fix:
- _commitTranscript accepts an optional prefixOverride parameter.
- _transcribeBlob snapshots _prefix into a local before the await.
- Pass the snapshot through to _commitTranscript.

Add a Settings → Voice → 'Append dictation to composer' toggle (default
ON) so users can opt into replace mode if they prefer the old buggy
behavior. The bug fix changes the default to append, matching what the
browser SR path always did.

i18n: new keys dictation_append_label / dictation_append_desc added to
all 14 locale blocks in static/i18n.js (English fallback where no native
translation exists).

Files: static/boot.js (+43/-7), static/index.html (+7), static/i18n.js (+30).

* fix(dictation): snapshot _prefix in onstop, not inside _transcribeBlob

First attempt at the append-mode fix took the snapshot inside
_transcribeBlob via 'const prefixSnapshot = _prefix'. By the time
_transcribeBlob runs (awaited from recorder.onstop), the sync block
above has already called _setRecording(false), which clears _prefix
inside _setRecording's !on branch. So the snapshot was always '' and
the append path fell through to the replace branch.

Move the snapshot into recorder.onstop, before _setRecording(false)
runs. Pass it as a parameter into _transcribeBlob so the call site
doesn't need to re-read the (now-cleared) module-level _prefix.

No behavior change for the browser-SR path — sr.onend still builds
the merged string into a local variable before invoking
_setRecording(false), and was already correct.

* fix(dictation): trim comments to fit test_590_recording_stops_before_transcribe

test_bugbatch_apr2026.py::test_590_recording_stops_before_transcribe slices
700 chars from 'recorder.onstop' and asserts that _setRecording(false)
comes before _sendRawAudio(/_transcribeBlob( inside that window. With
the verbose 6-line comment block from the previous fix, the actual
call sites landed at offsets 834/890 — past the 700-char window — so
the test's .find() returned -1 and the assertion failed mechanically.

Also: the verbose comment in _transcribeBlob contained the substring
'recorder.onstop', which made the test's first .find() land in the
comment instead of the actual handler, returning a wrong onstop_start.

Trim both comment blocks to single lines that fit within the test's
window and don't contain the literal 'recorder.onstop' substring.
Functional behavior unchanged from the previous fix; this is purely
a comment-shape adjustment to satisfy the brittle test.

* fix(tests): anchor test_590 on handler definition + widen window per review

The reviewer's fix-spec for test_590_recording_stops_before_transcribe
was more thorough than my single-shot comment trim:

  (1) Reword the _transcribeBlob comment so it doesn't contain the
      literal 'recorder.onstop' substring.
  (2) Make the test robust to the handler growing: anchor on
      'recorder.onstop=async' instead of 'recorder.onstop', and
      widen the fixed 700-char window to 1200.

Applied both. With these changes the test passes regardless of how
much comment text gets added in or around the handler in the future.
Also restored the verbose explanatory comments in both locations
(the previous a7f84027 had trimmed them to fit the brittle 700-char
window — that workaround is no longer needed).

Backward-compat check: the new test logic also passes against
origin/master (verified locally), so this change doesn't regress
the existing green-master invariant.

* fix(dictation): preserve user's mid-transcription edits in async server STT

Greptile flagged this as a 4/5 concern on the append-mode branch:
the async server-STT path appended the dictated text to the stale
snapshot (`_prefix` captured at recording start), which clobbered
any keystrokes the user made during the ~1s transcription round-trip.

Before this fix, with append mode ON:
  User typed before mic:    'Hello, '
  User typed during STT:    'Hello, from K'  (typed while waiting)
  Dictated text:            'world'
  Result:                   'Hello, world'   (lost 'from K')

After this fix:
  User typed before mic:    'Hello, '
  User typed during STT:    'Hello, from K'
  Dictated text:            'world'
  Result:                   'Hello, from K world'  (live state preserved)

The fix prefers live `ta.value` as the append base when the textarea
is non-empty, falling back to the `prefixOverride` snapshot only
when the textarea is genuinely empty (the original empty-composer path
where nothing to race with).

Adds a regression test in test_raw_audio_upload.py that asserts the
new `ta.value ||` fallback structure on `_commitTranscript`.

* fix(dictation): don't resurrect cleared text on async transcription resolve

Greptile P1 (re-reviewed at 20:07 UTC): the previous race fix
(`ta.value || (prefixOverride ?? _prefix)`) treats an empty
`ta.value` as falsy and falls back to the recording-start snapshot.
If the user explicitly clears the textarea while server transcription
is in flight, the cleared text reappears with the dictated text
appended — the user's intent is violated.

Fix: when prefixOverride IS passed (server-STT path), trust live
ta.value UNCONDITIONALLY — even when empty. The fallback to _prefix
only fires when no prefixOverride is given AND ta.value is empty,
which is purely defensive (no current caller hits this branch — the
browser-SR path commits inline in sr.onend without calling this
function).

Concrete repro:
  User typed before mic:    'Hello, '
  User clears textarea during STT
  Dictated text:            'world'
  OLD (0239bb4e) result:    'Hello, world'  (cleared text resurrected)
  NEW result:               'world'        (user intent respected)

Greptile's suggested one-liner:
  const base = prefixOverride !== undefined ? ta.value : (ta.value || _prefix);
Applied with expanded inline comment documenting the three concerns
this function balances (race, clear-during-STT, defensive fallback).

* Release exp-v0.52.49: preserve composer text on server STT commit (#5895)

---------

Co-authored-by: Konstantin M <kopamedve@gmail.com>
Co-authored-by: nesquena-hermes <agent@nesquena-hermes>