Fix #958: highlight IPv6 + allow editing built-in keyword rules

[binaricat]

Summary

Closes both halves of #958 in a single PR:

1. bug: the built-in "URL, IP & MAC" highlight rule only matched IPv4, so IPv6 addresses (the issue screenshots show 2001:11:22:33::5 and fe80::d2dd:bff:fe79:f2bb) were never highlighted.
2. feature: "would like to be able to modify the default rules" — built-in rules are now fully editable from the settings UI.

Changes

1. IPv6 regex

Added IPV6_HIGHLIGHT_PATTERN in domain/models.ts and merged it into the existing 'ip-mac' default rule's patterns array (per the issue follow-up: "fold it into the existing rule"). Covers:

• Full 8-group form: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
• Mid-compressed: 2001:11:22:33::5 (issue example 1)
• Link-local compressed: fe80::d2dd:bff:fe79:f2bb (issue example 2)
• Loopback: ::1
• All variants of :: at the head / middle / tail

Out of scope for this first pass (intentionally omitted, can become a follow-up issue): zone IDs (%eth0 suffix) and IPv4-mapped (::ffff:192.0.2.1). The carve-out is documented in the constant's comment.

No migration step needed for existing users: the existing "fill in missing default rules" logic in normalizeKeywordHighlightRules, combined with the customized change below, automatically refreshes the patterns of any ip-mac rule the user has not customized.

2. Editable built-in rules

Added an optional customized?: boolean to KeywordHighlightRule:

• customized unset / false (the default state): normalize overwrites with the latest default patterns/label (while preserving the user's color/enabled). This means any future pattern upgrade we ship (like IPv6 in this PR) automatically reaches every user who hasn't customized.
• customized set to true: normalize keeps the user's label/patterns/color/enabled exactly as saved.

Corresponding UI changes in SettingsTerminalTab.tsx:

• Built-in rules now also show the edit pencil icon. Clicking it reuses the existing AddCustomRuleDialog.
• The pattern input is upgraded from Input to Textarea, one regex per line. Built-ins like 'error' (which ships seven patterns) can now be viewed and edited in one go.
• Saving an edit to a built-in rule automatically flips customized to true.
• A customized built-in rule shows a small ↺ icon to restore the shipped label/patterns (the user's color is preserved).
• The footer's "Reset default colors" button is broadened into "Reset built-ins to defaults".

Tests

New domain/keywordHighlight.test.ts — 6 tests:

• IPv6 regex matches both issue examples + loopback + full form
• IPv4 / MAC still match (no regression)
• Plain hex blobs are not falsely matched
• normalize migrates a legacy non-customized ip-mac rule (without IPv6) up to the new defaults
• normalize keeps user-edited values verbatim when customized: true
• normalize leaves user custom rules untouched

npm test → 808 pass / 0 fail / 3 skipped

npx tsc --noEmit is clean on the changed files (other pre-existing repo-wide TS errors are unrelated to this PR).

Test plan

• Unit tests cover both the regex and the normalize behavior
• Manual: open a terminal where SSH output contains 2001:11:22:33::5 and fe80::d2dd:bff:fe79:f2bb, confirm both are highlighted
• Manual: Settings → Terminal → Keyword highlighting, hover the built-in "URL, IP & MAC" row to see the pencil; clicking it should show all 4 patterns (URL / IPv4 / IPv6 / MAC). Editing one and restarting netcatty should keep the edit
• Manual: click the ↺ icon on a customized built-in rule — patterns restore to defaults; the custom color is preserved
• Manual: the footer "Reset built-ins to defaults" button restores every built-in in one click while leaving custom user rules alone

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