Desktop app: make the built-in tab discarding actually kick in (it's set to 48h)

The desktop app keeps every open file tab fully loaded, each in its own renderer process. With ~12 file tabs open (design system libraries, UI kits, product files), Figma uses 7–9 GB of RAM on my 18 GB MacBook (M3 Pro). That pushes macOS into 8 GB of swap, slows the whole machine and drains the battery. Quitting and reopening doesn't help, because all restored tabs load again right away.

The usual advice (close unused tabs, restart, clear cache) works, but I lose my tab setup, and the tab bar plus the native menu bar are the main reasons I use the desktop app instead of the browser.

The interesting part: the desktop app already has tab discarding. Its config is delivered as the dynamic config desktop_tab_discarding and ends up in ~/Library/Application Support/Figma/settings.json:

"desktop_tab_discarding": {
  "default": {
    "tabInactivityThreshold": 172800000,
    "discardCheckInterval": 3600000,
    "minTabThreshold": 5,
    "minNonDiscardedTabThreshold": 3,
    "gracePeriod": 1800000,
    "inactiveDurationForGracePeriod": 14400000
  },
  "memoryTierOverrides": {}
}

A tab is only discarded after 48 hours of inactivity, checked once per hour. In practice it never kicks in during a normal workday.

Request:
1. Lower the default tabInactivityThreshold to something like 20–30 min, the way Chrome's Memory Saver and Safari do it. You could also use the existing memoryTierOverrides to be more aggressive on 8–18 GB machines.
2. Even better, expose it as a setting: Preferences → "Unload inactive tabs after [15 min / 30 min / 1 h / 4 h / never]", plus a right-click → "Unload tab" on the tab bar.
3. Optionally show unloaded tabs dimmed, so users know a click will reload them. And fixed groups don´t unload tabs.

The mechanism (discard, then reload on click) is already built and tested. This looks like a config change plus a small settings UI, and it would cut memory use by several GB for anyone who keeps many tabs open.