fix(electron): detect available session path launchers

[Pleasurecruise]

Related issue

Fix #138

Problem / pressure

The Session header advertised every platform-compatible path launcher, even when the corresponding editor or URL protocol was not installed on the local machine. Selecting one then failed only after the user tried to launch it.

Summary

• Add a validated Electron IPC probe for path-launcher availability.
• Resolve command candidates through the user's cached shell PATH, platform executable rules, known absolute fallbacks, macOS application lookup, and registered URL protocols.
• Filter the Session header launcher button and menu to the launcher IDs confirmed by the main process.
• Keep invalid stored custom launcher templates out of probe requests.

Before / after

Before	After
Shows all supported editors, even if they are not installed.	Shows only editors installed on this machine.

Test plan

• node --disable-warning=MODULE_TYPELESS_PACKAGE_JSON --experimental-strip-types --test src/main/services/local-path-launcher-core.test.mjs — 5 tests passed.
• Electron package test run reached 45 passing tests; the unrelated Loro data-plane test could not start because this worktree lacked the linked zod dependency.
• The focused components test could not start because this worktree lacked its linked Vitest module.
• No further tests were run after the final diff-only review, per the author-side user's instruction.

Context handoff

Instructions for reviewing agents

• Review focus: Check the probe boundary in app-ipc.ts, cross-platform command discovery in local-path-launcher-service.ts, and filtering lifecycle in session-chat-interface.tsx.
• Decisions to challenge: Verify that a five-minute per-command/protocol cache is the right balance for multiple mounted Session surfaces.
• Plausible failures / evidence gaps: Windows and Linux discovery paths were reviewed but not exercised on those operating systems, and the components test runner was unavailable in the worktree.

Authoring context

• User goal / directives: Make the Session header dynamically show only locally available editors and keep the uncommitted change small and necessary.
• Constraints / non-goals: Do not broaden launcher behavior, add cloud dependencies, or retain temporary file moves and duplicate tests.
• Risk-bearing decisions: Availability is determined in Electron main from commands, fallbacks, macOS application lookup, and registered URL protocols, then cached for five minutes.
• Destructive or irreversible behavior: The change only probes local availability and filters UI options; it performs no destructive or irreversible action.
• Deliberately not done or tested: No further tests were run after the author-side user requested diff-only review; Windows and Linux runtime probing was not exercised.
• Unknowns / confidence: Confidence is high in the IPC and filtering flow; residual risk is platform-specific command and protocol discovery behavior.

[Leeeon233]

This is a good feature! Thanks