feat(downloads): resumable downloads with pause that survive app restarts

[ghostcoder42]

• downloading tasks have a Pause button; pausing parks the native task at a clean boundary (pauseAsync) and the row shows Paused with Resume/Delete — the progress bar resumes where it stopped
• Retry/Resume continues from the bytes already on disk (resumeAsync + Range) instead of restarting from zero; after an app restart the partial file's size rebuilds the resumable (the native layer truncates to it and sends Range: bytes=-)
• user-paused tasks stay paused across restarts; tasks that died with the process come back as errored ("Interrupted"); the orphan cleanup now spares partial files of unfinished tasks (hydrate restores tasks before reconciling, so the resume points survive)
• when resuming is impossible (expired source URL, range rejected), retry falls back to a fresh download with a re-resolved source
• cancelling an errored/paused task deletes its partial file too
• the post-page download button is store-driven (same story as the library row and tile badges), so its retry/resume works; the long-press menu offers "Resume download" for errored/paused tiles

[github-actions]

💯 Test Coverage

Lines	Statements	Branches	Functions
	58.08% (1106/1904)	51.5% (635/1233)	47.48% (274/577)

😎 Tests Results

Tests	Skipped	Failures	Errors	Time
327	0 💤	0 ❌	0 🔥	14.007s ⏱️

👀 Tests Details • (58%)

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	58.08	51.5	47.48	58.5
src/app/(app)	4.83	3.01	4.2	4.95
_layout.tsx	0	0	0	0	20–103
library.tsx	0	0	0	0	22–448
src/components	75	89.39	54.54	75.67
video-tile.tsx	86.36	93.87	100	85.71	71, 76–77
src/lib/download	62.43	58.87	55.55	63.1
download-video.ts	73.13	62.92	58.33	74.01	43–47, 60, 64–69, 128–131, 195, 214, 241, 277–287, 310, 315–323, 339–350
index.ts	39.68	38.88	53.33	40	49–72, 94–97, 112–115, 129–161
src/lib/hooks	58.42	43.75	71.73	57.64
use-video-actions.ts	37.25	10.34	72.72	31.7	41–42, 57–93, 103–122
use-video-download.ts	68.96	41.02	100	69.23	57, 62–63, 90–97
src/lib/stores	96.23	94.44	94.93	97.4
active-downloads-store.ts	95.31	88	100	100	144–165
downloaded-store.ts	92.59	100	85.71	91.3	46–47