Docs: align terminology with glossary

[naltatis]

refs evcc-io/evcc#34718, follows evcc-io/evcc#34787 and evcc-io/evcc#34836

Wording pass over the hand-written EN and DE pages against the shared glossary and the renamed UI labels. Terminology only, no restructuring. Blog posts, CLI reference and generated device pages are untouched.

• EN: feed-in → grid export (feature name "Feed-in priority" kept), loadpoint → charging point, wallbox → charger, PV → solar, SoC → charge level, battery storage → home battery, HEMS → external limit, UK spelling
• DE: Speicher → Hausbatterie, Prognose → Vorhersage, Produktion → Erzeugung, Ladeplan → Ladeplanung, Solar/Sonne → PV, Einspeisetarif → Einspeisevergütung, Sponsortoken, SoC → Ladestand
• EEBus → EEBUS in both languages
• Quoted UI labels checked against i18n/{en,de}.json (e.g. PV-Anteil, Mindestladung, Charging points & heaters)
• Meters nav label and its link texts now "Solar, Home Battery, Grid, Meters" / "PV, Hausbatterie, Netz, Zähler"
• DE FAQ forecast heading uses the English anchor #solar-forecast

TODO

• DE FAQ savings section still says "Sonnenenergie", matching the current UI footer
• optimizer page mentions charging plans, which mixes the two concepts

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