fix: restore mobile session navigation and add machine menu icon

[renyuanz]

Tapping the mobile task title opens the session switcher again. Replace the top-right session counter with a Monitor icon that opens Machines / Hub, and show the session position/count in the switcher's fixed header so scrolling cannot clip it.

Preserve the 44px single-row header, title swipe and long-press interactions, existing design tokens, and host/Hub actions. Update connection documentation and dependent navigation tests.

Validation:

• App TypeScript check and git diff --check passed.
• Latest targeted pnpm e2e:test run: 11 passed using container Chromium.
• Android touch emulation in light and dark themes: 320×568, 344×882, 390×844, 757×840, 840×757, 1024×768, 882×344, then fold back to 344×882 without reloading.
• Verify non-overlapping single-row controls, title/session and Monitor/machine actions, title swipes and long presses, current-session preservation through resize, scrolling and selecting rows, fixed count visibility, and reaching menu items on short screens.
• Regression coverage includes terminal creation/closure, active-row positioning, host metrics, and encrypted Hub navigation. Reviewed narrow, unfolded, and short-landscape screenshots.

These checks emulate Android touch in the container browser; they do not constitute physical foldable-device verification.