chore: delete TODO.md, superseded by issues (#208)

[BernardJen]

Closes #208.

Why deleted rather than kept with a header

#208 offered both. Deleted, because a stale roadmap that announces its own staleness is
still a file people open — and every claim in it now needs checking against reality before
it can be trusted, which is the opposite of what a roadmap is for. The content stays in git
history.

It was actively misleading, not merely absent:

• listed the screensaver as a completed "DVD-style bouncing logo" — there are now 30;
• listed the release pipeline as publishing to a separate input-viewer-releases repo, which
  was removed;
• listed as pending four things that have shipped.

TODO said pending	Reality
Cursor shake detection	shipped in 2.4.0; shake handling in 2 source files
Audio controls	shipped in 2.4.0
Touchscreen controls	shipped in 2.4.0
CI testing, unit + integration tests	shipped; 410 test cases across 31 files

Not a bare delete

Seven items in it were genuinely unimplemented and recorded nowhere else. I verified each
against src/ rather than trusting the file — for instance the only thumbnail references in
the source are the no-signal reference panel, which is a different feature from TODO's "live
preview of all inputs", and there is no autoSwitch or mock handling anywhere.

Those are preserved in #239 before this deletion — a thumbnails panel, auto-switch mode,
multi-view, picture-in-picture, keyboard customisation, layout animations, --mock /
--no-signal flags, and the Tauri migration note. So "issues are the source of truth" is now
true rather than merely asserted.

I also recorded the shipped-but-listed-pending items in #239, so nobody re-implements them from
the git history.

Acceptance criteria

• TODO.md removed
• If removed, CONTRIBUTING.md and README.md checked for links — nothing in the repo
  references it: no markdown, workflow, package script or source file
• (n/a — kept branch not taken)

Verification

• Branched from main (ce099d9).
• Deletion only: 1 file, 91 deletions, no other change.
• Link check run across *.md, *.yml, *.yaml, *.json, *.mjs, *.js excluding
  node_modules — no hits.

Note

This was the last open part of #204, the documentation umbrella. Its changelog half was
#205 and its two wrong-documentation halves were #238, so #204 can close once this merges.