fix(flow-chat): report and navigate every search keyword occurrence

[bobleer]

Summary

FlowChat's in-chat search deduplicated matches by dialog turn: a session containing many occurrences of the keyword reported a single result ("1/1"), and the next/prev controls skipped every other occurrence inside a turn. This PR makes every occurrence of the query its own navigable match, teaches DOM navigation to land on the exact Nth occurrence, and adds a faint highlight for the non-current occurrences in the focused text root.

• useFlowChatSearch: buildFlowChatSearchMatches no longer keeps only the first match per turn; it enumerates every non-overlapping, case-insensitive occurrence in every searchable source and records its occurrenceIndex.
• flowChatSearchDom: new findFlowChatSearchTextRanges returns all match ranges in document order (still tolerant of Markdown splitting a match across text nodes); setFlowChatSearchHighlight now maintains a second ::highlight registry (bitfun-flowchat-search-match) for non-current occurrences.
• VirtualMessageList.scrollToSearchMatch accepts occurrenceIndex, picks the Nth rendered range (clamped, since raw Markdown can contain occurrences the rendered DOM does not), highlights the current one strongly and its siblings faintly.
• ModernFlowChatContainer threads occurrenceIndex through to the list so stepping between occurrences inside the same message re-triggers navigation.

Type and Areas

Type: bug fix

Areas: web UI (FlowChat search)

Motivation / Impact

Searching a session for a keyword now reports the true occurrence count (e.g. "3/17"), and next/prev/Enter walk through every occurrence — including several inside one long model reply — instead of jumping one-per-turn. All occurrences in the current message are visibly highlighted, with the active one emphasized.

Verification

• node node_modules/typescript/bin/tsc --noEmit in src/web-ui: clean.
• vitest run src/flow_chat: 148 files, 1156 tests passed, including new cases for multiple occurrences in one source, per-item matches within one turn, non-overlapping counting, and document-order multi-range DOM lookup.

Reviewer Notes

The occurrence count comes from raw message content while navigation resolves against rendered text, so Markdown syntax (e.g. link URLs) can make the raw count exceed the rendered one; navigation clamps to the last rendered occurrence in that case. Scroll/expand retry machinery in scrollToSearchMatch is unchanged, consistent with FLOWCHAT_SCROLL_STABILITY.md.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.