[Feature] Reduce CPU/RAM usage and binary size

[thangnm93]

Problem Statement

A sweep of the codebase and release pipeline found avoidable runtime overhead and bundle dead weight:

• Keyword-highlight rules were recompiled (one RegExp per rule) on every terminal build, with the same conversion chain duplicated across three widgets
• SessionProvider.setActive notified the whole tree even when re-clicking the already-active tab
• SSH agent messages were built with double list spreads in the auth hot path
• local_auth was declared but never imported anywhere, pulling native plugins into the macOS/Windows bundles
• MesloLGS NF shipped 4 faces (~9.8 MB of the 11 MB font folder); italic faces are rarely hit in terminals
• Release builds shipped Dart debug symbols inside the AOT snapshot

Proposed Solution

• Memoize compiled xterm keyword rules in SettingsProvider (cache keyed on rules-list identity + the enabled flag; rules are only ever replaced wholesale)
• Equality guard in setActive
• Build agent messages with a direct buffer write
• Drop the unused local_auth dependency
• Drop MesloLGS NF Italic / Bold Italic (−4.8 MB per bundle; the engine falls back to a synthetic slant)
• --split-debug-info on all desktop release builds, with per-platform symbols zips attached to each release (flutter symbolize -d <symbols> to read crash traces)

Alternatives Considered

• --obfuscate on top of split-debug-info — minimal extra size win, complicates every crash report; skipped
• Dropping the smaller Powerline fonts (~1.3 MB) — they are user-selectable font options; removing them is a feature regression for little gain
• Demand-driven SOCKS connection polling — rejected after verification: PortForwardProvider.setConnections already equality-guards, so the idle 2 s sample is negligible

Additional Context

Implemented in #61. Several findings from the initial scan were rejected after code verification (recording buffer is already bounded by the 500 ms flush timer; the suspected RDP listener leak is held by the session and GC'd with it).