docs: add post-0.1.0 usability backlog and app icon

Collects the items found while actually using 0.1.0. Two clusters dominate:
state that does not survive restart (splitter, zoom, account selection) and
actions reachable only by memorized keys.

The plan is open by design rather than a fixed release scope. Numbering is
stable so notes referring to an item keep meaning the same thing.

Decisions recorded while triaging:

- UI state goes to its own file, not qtmaildir.conf. That file is hand-edited
  and rewriting it on exit would eat comments QSettings does not preserve.
- Auto-mark-read stays off the undo stack. An explicit toggle_unread action
  already exists, so Ctrl+Z need not undo an action the user never took.
- Zoom is Chromium's, not ours. No setZoomFactor call exists in src/, so
  persisting it means taking ownership of zoom first.

Icon is a tag rather than an envelope, since tagging is the core interaction
and an envelope would not distinguish it from any other mail client. Verified
legible at 16x16, which is where it is mostly seen.

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