fix(web): usable past-conversation list, and search that finds past sessions

Two home-screen reports from @jordan8037310, both about history that is
present but unreachable.

#260 — "Resume Conversation" rendered 4 rows, then a button that appended
every remaining row into a `max-height: 240px` box, so 35 conversations
landed in a four-row scroll well with no ordering or filtering. Rendering
now goes through `_renderHistoryList()` over a cached corpus: 10 rows to
start, Show more/Show less that grows and shrinks the box (the height cap
is class-driven, `.history-list.expanded`), plus a filter box (name,
folder, #case label, prompts), a sort control (recent / name / folder,
pinned rows still first) and a shown-of-total count. A filter implies
expansion, so every match is visible, and the whole header hides as one
unit while a federated search is active. The A-Z sort keys off the same
string the row renders, since most rows are transcript-backed and carry
no session name at all.

#261 — the search box could not match a past project by folder name:
`harvestSources()` built its session corpus from the live in-memory map,
while past sessions come from `/api/sessions/unified` (lifecycle log +
transcript scan). Folding that scan into the request path would have cost
the search its no-filesystem-reads property, so the corpus arrives via a
bounded snapshot instead: `session-history-index.ts` is published as a
side effect of `/api/sessions/unified` (the home screen fetches it on
open, which is the same screen the search box lives on) and rebuilt
fire-and-forget, single-flight and TTL-guarded when a search finds it
stale. A result for a closed session now resumes the conversation rather
than selecting a tab that no longer exists, and is badged RESUME.

The snapshot is stored unscoped with a per-row owner and re-filtered
through canAccessOwned() on read, so multi-user sees exactly what
/api/sessions/unified exposes: own sessions only, host-wide transcript
history admin-only. Live rows are harvested first and win the dedupe.

Verified end-to-end against a real instance with 60 past sessions: cold
process answers its first search without history and its second with it;
folder-name queries return resume targets; clicking one posts the right
resumeSessionId + workingDir.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/index.html

@@ -397,7 +397,27 @@
               </div>
               <div class="search-results" id="searchResults" hidden></div>
             </div>
-            <h3 class="history-title" id="historyTitle">Resume Conversation</h3>
+            <div class="history-header" id="historyHeader">
+              <h3 class="history-title" id="historyTitle">Resume Conversation</h3>
+              <span class="history-count" id="historyCount" data-i18n-skip></span>
+              <div class="history-controls">
+                <input
+                  type="search"
+                  id="historyFilter"
+                  class="history-filter"
+                  placeholder="Filter…"
+                  autocomplete="off"
+                  spellcheck="false"
+                  maxlength="100"
+                  aria-label="Filter past conversations"
+                />
+                <select id="historySort" class="search-select history-sort" aria-label="Sort past conversations">
+                  <option value="recent">Recent</option>
+                  <option value="name">Name A–Z</option>
+                  <option value="folder">Folder A–Z</option>
+                </select>
+              </div>
+            </div>
             <div class="history-list" id="historyList"></div>
           </div>
           <p class="welcome-hint">Or click Run to start</p>