feat: diff images, secret snippets, retagging, and a tooltip sweep

[mindaugaskasp]

Several related pieces of work; they share files (diffStore, DiffViewer), so they land together rather than as artificially split commits that would not build in between.

Diff image export

• Scroll-and-stitch: a diff taller than the pane is captured a viewport at a time and joined in main from raw BGRA rows (no new dependency). A single capturePage can never exceed the pane — Monaco keeps no offscreen lines in the DOM.
• Wait for Monaco's diff WORKER before the shutter. Counting frames photographed the files with no highlights, which read as "no differences".
• Select lines in either pane to export just those; the selection follows the pane last used, and is hidden from the picture.
• Ceiling is configurable (Settings -> Limits) in SCREEN pixels, so the same diff captures the same amount on any display scale. Main keeps its own pixel/height backstop — it must not trust a renderer loop.
• Preview data URL is downscaled; full resolution stays in main for copy/save, and the PNG is encoded lazily.

Secret snippets

• Mark a snippet secret: contents never render in the editor, hover card or launcher, but still copy in full. Monaco is unmounted while masked, so the plaintext is not in the DOM at all.
• Fixed-width mask (a length-preserving one would leak the size), and a secret is never decrypted just to be previewed.
• Masking sits on top of the encryption every snippet already has.

Saved diffs

• Fix "Untagged" on a tagged diff: the format was stored both on entry.format and injected into tags, then subtracted back out, so a user tag matching the format vanished.
• Retag a saved diff from its row (setTags had no caller).
• Stop the "both sides are identical" banner flashing on every diff opened: Monaco returns null until its worker lands, and null was folded into zeros.

Tooltips

• Interactive controls move from native title, which Electron often never draws, to the app's own tooltip; the search toggles now explain what they do. Long tips wrap and are clamped inside the window.
• Icon-only controls gained accessible names; e2e locators that found controls by title updated to match.

Verified: npm run check (1012 tests) and the full Docker e2e suite (139 passed, 2 skipped).