web-ui: hover popover labels the session top-bar icon buttons (#511)

The session top bar's tool icons (crons, files, apps, skills, memory, keychain) relied on the native title attribute, which is slow to appear and invisible to keyboard users. Each button now gets a styled tooltip pill: right-aligned to avoid clipping, no show delay, and shown on keyboard focus as well as hover. The native title is replaced with aria-label plus a role=tooltip span.

Depends on #486 — do not merge until it lands.

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