🤖 Draft merge: removeDraftBlock follow-ups (indentation as a boundary, scan cost)

[ThomasK33]

Follow-up from PR #5577 (#5567, whole-block draft matching). Two normal-review findings on its head 7037a70e83, deferred because the PR's review budget was spent. Both are in removeDraftBlock (src/common/utils/composerDraftText.ts).

1. Indentation counts as a block boundary. whitespaceRun skips the same-line indentation before it counts the blank line, so in alpha\n\n yes\n\nomega an indented code line yes matches a send yes. mergeWrite can then take that line out of a window's write as a stale copy, and observeSends marks it inUnsavedText. Before 🤖 fix: match a send's text in the draft merge only as a whole block #5577 the middle check (\n\n<block>\n\n) did not match it, but the end checks (removeSentText) did. Fix: only whitespace on separator lines may sit between the block and the blank line. Indentation or trailing spaces on the block's own line must prevent the match.
2. Per-occurrence scan cost. The loop visits every occurrence of the block. A near-limit draft (MAX_DRAFT_JSON_CHARS) of millions of a with a send a and no whole-block match runs the loop millions of times, for each pending send in mergeWrite, on the backend event loop. Fix: search only separator-delimited candidates (for example, look for \n + block and check the edges), so the work is bounded by line count.

Both need the send's text to equal a line of the user's draft (1) or a pathological draft (2). Neither was reported by a user.

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[ThomasK33]

3. Quadratic trailing-whitespace regex (from 🤖 fix: match a send's text in the draft merge only as a whole block #5577's last commit 7037a70e83). joinAround uses parts.before.replace(/\n\s*$/, ""). When the text before a removed block ends in one long whitespace run with many line breaks, the regex retries from every line break: the final readiness check measured about 6.8 s for 100,000 consecutive \n (the old trimEnd() took 0.05 ms). Normal drafts are unaffected (20,000 paragraphs: 0.3 ms). Fix: walk back over the trailing whitespace, then cut at the first \n in that run. Same kind of problem as item 2.

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