feat(compose): transform the markdown buffer for the toolbar, item 123

MarkdownFormat, task 8 of the compose-and-send plan. Three free functions
over (text, selection start, selection end) returning the new text and the
selection that follows it, so the grammar is tested without a widget.

Three gaps in the plan's draft, each now pinned by a test checked against the
mutation that breaks it:

- QString::lastIndexOf INCLUDES the position it is given, so quoting with the
  cursor at the end of a line found that line's newline and quoted the
  FOLLOWING one. The draft's fixtures never placed a cursor there.
- A backwards selection was normalised but never tested, so the swap was
  unguarded; a right-to-left drag is an ordinary gesture and Qt reports the
  anchor after the cursor. normalise() now swaps and clamps in one place.
- A blank line inside a quoted range produced "> " with trailing whitespace,
  which editors and mail clients strip anyway. It is written bare.

Two further defects came out of review:

- quote()'s selectionStart was unasserted for any block not starting at line
  zero. Hardcoding it to 0 passed all nineteen tests, because the one test
  naming the property quoted the first line, where right and wrong coincide.
  A wrong selection there means a second press quotes a line the user never
  selected, and a following Bold bolds the wrong text.
- A selection splitting a surrogate pair split the character across the
  inserted tokens, leaving invalid UTF-16. Not reachable from the toolbar,
  where arrow keys and mouse hit-testing both move in whole clusters, but
  reachable by any code computing a position arithmetically. normalise()
  nudges off a low surrogate; a collapsed cursor moves back on both ends,
  since widening would turn "insert an empty pair here" into "wrap the emoji".

The buttons stack rather than toggle: a second Bold press gives ****this****,
and a second Quote press nests. That is what the spec specifies, and the
preserved selection exists so a second press can apply a SECOND token. A
toggle was built during this task at the user's request and reverted on
finding it contradicts the spec at two sites; it is recorded as backlog item
135, where the unanswered question is what replaces bold-then-italic.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LoaLBowZ6w1JNx6SEhDP1L

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