Transcript: double-click drags keep selecting whole words

[dfrnoch]

• Double-clicking chat text selected the word, but the selection was stored as a normal drag starting at the word's first letter. Any mouse move before release, even slight tremor, re-resolved it letter by letter from there to the pointer. The word collapsed to a few letters and the gesture behaved like a plain drag. Triple-click had the same problem.
• The selection now remembers it started as a double- or triple-click. While the drag lasts, the pointer end snaps to word boundaries (double-click) or whole text elements, i.e. paragraphs, list items and code lines (triple-click). The clicked unit stays selected in either direction, matching the composer and native text fields.
• Also applies when the drag auto-scrolls far enough that the starting row leaves the virtualized list.
• Rendered markdown in the file preview uses the same selection code and gets the fix too.
• Worth a look: this touches the same render.rs mouse-down listener as open PR Fix transcript selection leaking through popups #556. The overlap is small, only the begin_with_span call sites.

Test plan

• markdown::render::tests::code_block_lines_participate_in_text_selection now moves the pointer after a double-click. Fails without the fix ("s" instead of "selectable"), passes with it
• selection::tests::double_click_drag_extends_by_whole_words: tremor keeps the word; dragging forward, backward and into the next element extends by words
• selection::tests::triple_click_drag_extends_by_whole_elements
• selection::tests::word_drag_snaps_after_the_anchor_virtualizes
• cargo test --locked -p zeron-ui --lib -- --test-threads=1
• cargo clippy -p zeron-ui --all-targets: no new warnings in touched code
• Manual: double-click a word in an agent reply, then double-click and drag across words and messages

Video preview (This PR vs latest release build)

8mb.video-ci2-LPjtURm7.mp4

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