Make Wayland capture honest and replace the broad input group with least privilege

Phase 3 of the improvement plan (PR 10 of the sequence): honest Wayland
behavior and a least-privilege alternative to the broad `input` group the
audit flagged as a keylogging surface.

Least-privilege input access:
- packaging/linux/common/60-stepforge-input.rules grants the ACTIVE session
  read access to MOUSE devices only (ID_INPUT_MOUSE, excluding
  ID_INPUT_KEYBOARD) via a systemd uaccess ACL — session-scoped, device-
  scoped, and never keyboards. This replaces `usermod -aG input`, which grants
  every input device (keyboards included) to the user permanently.
- scripts/linux/enable-click-capture.sh installs it opt-in: it prints the exact
  rule, requires confirmation, and documents the security tradeoff. It never
  runs the broad-group command.

Honest trigger reporting:
- New chooseCaptureTrigger() (app/platform/linux/diagnostics.js, re-exported
  from app/platform/index.js) maps a capability profile to the real trigger:
  per-click with a marker on X11+xinput; per-click WITHOUT coordinates or a
  marker on Wayland evdev (the platform exposes no pointer position); and an
  honest hotkey/interval fallback otherwise. It never promises per-click
  capture with coordinates on Wayland.
- platform:capabilities now includes the active trigger for the machine and
  the user's fallback setting, for the diagnostics UI.

Docs:
- GETTING_STARTED_WITH_LINUX.md no longer instructs every Wayland user to join
  the `input` group; it presents that as a warning, documents the least-
  privilege script, corrects the capability table (per-click is opt-in, mice
  only; hotkey/interval is the default Wayland trigger), and points at
  Settings → Diagnostics.

Tests: trigger decisions for X11/xinput, Wayland evdev (no coordinates/marker),
Wayland fallback (interval/hotkey with an honest note), the platform facade
wiring, Windows; the udev rule (mouse-only, excludes keyboards, uaccess); the
opt-in enable script (confirms, installs the rule, never runs usermod -aG
input as a command); and docs guards. 289 unit tests pass; startup smoke and
click self-test unchanged (stream, markers 3/3, burst 8/8).

Co-Authored-By: Claude Fable 5 <[email protected]>

app/platform/linux/diagnostics.js

@@ -96,4 +96,61 @@ function detectLinuxCapabilities({
   };
 }
 
-module.exports = { detectLinuxCapabilities, detectSessionType };
+/**
+ * Decide the honest capture trigger for a Linux capability profile. StepForge
+ * must never *promise* per-click capture with coordinates on Wayland, because
+ * the platform does not expose pointer position to apps. Returns the trigger,
+ * whether clicks carry coordinates, whether a marker can be drawn, and a
+ * user-facing note. `userTriggerPreference` is the capture.fallbackTrigger
+ * setting ('interval' | 'hotkey') used only when no click source exists.
+ */
+function chooseCaptureTrigger(capabilities, userTriggerPreference = 'interval') {
+  const caps = capabilities || {};
+  const click = caps.clickCapture;
+
+  if (click === 'x11-xinput') {
+    return {
+      trigger: 'click',
+      clickSource: 'x11',
+      coordinates: true,
+      marker: true,
+      note: 'Per-click capture with an accurate marker (X11 + xinput).',
+    };
+  }
+  if (click === 'evdev-x11') {
+    return {
+      trigger: 'click',
+      clickSource: 'evdev-x11',
+      coordinates: true,
+      marker: true,
+      note: 'Per-click capture via kernel input devices (X11, no xinput).',
+    };
+  }
+  if (click === 'evdev-wayland') {
+    // Wayland exposes button presses (via evdev, if permitted) but NOT pointer
+    // position, so a step is captured per click but without a marker. This is
+    // only reached when the user opted into the least-privilege device rule.
+    return {
+      trigger: 'click',
+      clickSource: 'evdev-wayland',
+      coordinates: false,
+      marker: false,
+      note: 'Per-click capture on Wayland has no pointer position, so no marker is drawn.',
+    };
+  }
+
+  // No global click source: the safe baseline is the user's chosen fallback.
+  const trigger = userTriggerPreference === 'hotkey' ? 'hotkey' : 'interval';
+  return {
+    trigger,
+    clickSource: trigger,
+    coordinates: false,
+    marker: false,
+    note: caps.isWayland
+      ? 'Wayland does not expose global clicks; recording uses your ' + trigger + ' trigger. '
+        + 'Screen sharing is requested once per recording via the portal.'
+      : 'No global click source available; recording uses your ' + trigger + ' trigger.',
+  };
+}
+
+module.exports = { detectLinuxCapabilities, detectSessionType, chooseCaptureTrigger };