Migrate timeline chrome to Lui_element_combine.buttons

[RCmerci]

Summary

Migrates the timeline page's remaining toolbar surfaces to the V.buttons glass-capsule composite, on lui 0c93a00 (logseq/lui#64) which adds a Menu action variant:

• buttons_action is now a variant — Press of {label; icon; text; on_press} | Menu of {label; icon; text; menu : Lui_elements.t list; on_dismiss}. V.buttons maps both onto the lui-level Lui_element_combine.action variants. V.buttons_action retains the old (label, icon, text, handler) signature so existing press callers are unchanged.
• New V.buttons_menu_action ~label ~icon ?text ~on_select entries builds a Menu action from the existing V.Menu.entry vocabulary (ids/roles/dividers/submenus preserved); Menu.entry_elements/to_elements were hoisted so Menu.create and the buttons wrapper share the same entry conversion.
• app/journal_header.ml: the top-right Account menu moves from V.Menu.create (which produced a chevron-down system row) to buttons_menu_action — a single-member glass capsule whose menu_trigger hosts the same five account actions. The destinations pair (Journals/Favorites) collapses from a V.Toolbar.group of two V.buttons into one V.buttons Press capsule.
• Behavior change: the destinations capsule no longer carries per-item selected state (the selected binding and navigation helper are deleted) — the pressed item navigates, and the destination itself is conveyed by the page content, matching how other buttons behave.
• logseq_journal.opam (+.locked) pin lui to 0c93a0000b1…; test/source_boundary_test.ml updated for the new pin and the header's new required symbols (V.buttons, V.buttons_menu_action in place of V.button/V.Menu.create).

Known preexisting failure on main (unchanged by this PR): source_boundary_test also requires "V.progress" in app/journal_timeline.ml, which has been absent since commit 316b159 removed it — the test fails on main too.

Depends on logseq/lui#64 — the pin should be repointed to that PR's merge commit once it lands.

Link to Devin session: https://app.devin.ai/sessions/3bce435625ba4bc086c09520e0920db5
Open in Devin Desktop: https://app.devin.ai/desktop/session/3bce435625ba4bc086c09520e0920db5?variant=devin
Requested by: @RCmerci

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

iOS sim verification — timeline chrome migrated to Lui_element_combine.buttons ✅

Verified on iPhone 17 sim (iOS 27.0), signed in, ocaml-sync-test graph, timeline loaded:

• Fused Journals+Favorites capsule navigates both directions; off-glyph taps inside a cell work (44pt hit area)
• Account capsule → native menu with Attachment settings / Diagnostics / Switch graph / Delete local graph copy / Sign out; "Diagnostics" opens the diagnostics sheet
• Capture capsule opens the composer; sheet/menu presentations dismiss cleanly

Caveat: taps within ~4pt of the capsule's rounded extremity can fall outside the cell — consistent with frame-based hit-testing, not a regression.

[devin-ai-integration]

Fused error+account capsule (053fea9) verified under real error state ✅

The two top-right controls were previously separate chrome slots each hosting its own V.buttons → two capsules. They now compose into one V.buttons cluster (error ! leads, account menu trails) mounted in the account slot; the error slot is only used when no account action exists.

Induced a live sync failure on the iOS sim (api.logseq.io blackholed via /etc/hosts — sims have no Airplane Mode):

• ! half → error-info sheet with "TLS connection setup failed"
• Person half → account menu opens (4 items; "Delete local graph copy" correctly hidden while local_deletion_available is false)
• Single glass capsule, no double ring; account-only case (no error) verified separately — bottom-bar capsules unaffected