feat: localize condition text and forecast weekdays (--language)

[KSxx]

Closes #1.

What

• --language <en|de>, falling back to LC_MESSAGES → LANG → English, per the design we settled on in Localize weather condition text (and forecast date formatting) #1.
• Translation lives in a new src/i18n.rs: a plain match, no i18n crate, matching the rest of the codebase's philosophy.
• Applied once, in icons::get_icon/get_icon_plain, so the Waybar tooltip and the structured JSON (and therefore the Omarchy panel) stay in sync — the same way icon_set already threads through both.
• Also localizes the daily forecast's weekday abbreviation (waybar.rs::short_day_name), since chrono's %a can't do that without the unstable-locales feature. Small fixed table in i18n.rs instead — same reasoning as the condition strings, no new dependency. I checked Panel.qml's own weekday label (Qt.formatDate "ddd"): it already follows the system locale on its own, so no QML change was needed there.
• Per your note on Localize weather condition text (and forecast date formatting) #1: language is not in manifest.json — the Omarchy panel reads it the same path units already takes, setting("language", "") threaded through buildCmd(), set (if at all) directly in shell.json.

Coverage

• i18n::every_condition_string_is_translated iterates icons::all_descriptions() (the same table icons.rs owns), so a new WMO code added later without a German entry fails the suite instead of shipping silently untranslated.
• structured::structured_output_is_localized and waybar::german_language_localizes_the_tooltip check both output surfaces end to end.
• Language::resolve has its own coverage: flag-wins, LC_MESSAGES over LANG, an explicit en* short-circuiting a later de* variable, unknown/missing locale defaulting to English.
• Manually verified against the real API with --language de, with LANG=de_DE.UTF-8/LC_MESSAGES (no flag), on both --output json and the Waybar tooltip — condition text and weekday abbreviations (Do 03, Fr 04, Sa 05) all correct.
• cargo test, cargo clippy --all-targets, cargo fmt, qmllint omarchy/Panel.qml all clean. The two too_many_arguments clippy warnings on build_tooltip/build_output predate this PR (verified against master); this just adds one more argument to each.

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