feat(mobile): adopt upstream video, image, and PDF media chain

[rynfar]

Adopts three upstream mobile media commits as a chain, hand-merged to keep Pylon's composer.

• #8919 video playback with native iOS controls
• #8959 native image and PDF previews
• #8978 upload attachments while composing

Together these give mobile inline video playback, a native image/PDF viewer, and
uploads that start as soon as you attach rather than at send — so queued
messages no longer hold their bytes hostage until delivery.

Carve-outs

Upstream moved videoMimeType into a new packages/shared/src/video.ts, but the
version they moved is the MIME-first one Pylon corrected in f9eb4230d. Hosts map
a TypeScript .ts source to video/mp2t, which turned source files into unnamed
black play tiles. The move is adopted; the body is Pylon's extension-first
implementation, and upstream's test that asserted the opposite ordering was
rewritten to Pylon's contract with the .ts regression cases added.

Pylon's composer diverges from upstream's and was preserved:

• ControlPill and ComposerToolbarButton call sites kept — upstream's hunks
  are written against ComposerActionButton, which Pylon no longer uses here.
• Stop stays outside the dictation ternary. Upstream's #8978 folds it into an
  else-if chain with Send; Pylon keeps an agent stoppable for the whole recording
  and transcription window.
• Send stays gated on voicePresentation.showsSend, so the dictation error phase
  does not strand the user without a send button.
• QuickQuestionModal, SessionResourcesModal, SessionAgentLiveActivityModal,
  and the agent-message sheet all sit in a block upstream replaces wholesale with
  a single VideoPreviewModal. Both sides are kept.
• Upstream's new upload-gating block redeclared three consts Pylon already has and
  read voiceInput about 800 lines before it exists. It was trimmed to the new
  uploadStates/attachmentBlockReason and reseated next to Pylon's canSend.
• NewTaskDraftScreen keeps Pylon's stricter workspaceRoot start condition and
  its richer unavailability messages, with the upload block reason folded in.
• useThreadOutboxDrain's dep array keeps Pylon's list plus serverConfigs;
  upstream's three extra deps name symbols Pylon's refactor removed.

Branding: NSPhotoLibraryAddUsageDescription and the composer docs arrived saying
"T3 Code" and "T3 Connect" and now say Pylon.

Behavior change worth naming

The collapsed send pill moves from disabled={!hasContent} to
disabled={!canSend}, matching upstream and the expanded toolbar. This is wider
than "attachment uploads": the pill previously rendered enabled and then no-opped
when projectCwd === null or provider admission was unavailable, because
handleSend early-returns on !canSend. It now reflects that state instead of
lying about it.

Verification

@t3tools/mobile typecheck clean, @t3tools/shared and @t3tools/web typecheck
clean, lint clean, 1163 mobile tests passing. No user-visible T3 Code string in
any added line.

Native rebuild and device pass still to come — this chain adds expo-video,
expo-document-picker, and three new Swift files, so the JS-only checks above do
not cover it.

Model: Claude Opus 5 in Claude Code.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	−6 B (−0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	+4 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.6 KiB	−10 B (−0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.2 KiB	57.2 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	+21 B (+0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	+12 B (+0.2%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.7 KiB	+9 B (+0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.1 KiB	58.1 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	10	10	0 (0.0%)	21	✅

Baseline: a509639 · PR result: d9a3bdc · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.5 KiB
• Claude decoded thread snapshot: 110.2 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.