refactor: drop formobj, use PageReference for preview Back

[tkuhn]

Summary

Replaces the `formobj` query-param + session `formMap` workaround with Wicket's native `PageReference` for the "Back from preview" flow (see #373).

What changed

• `NanodashSession.PreviewNanopub` now holds a `PageReference` to the source `PublishPage`.
• `PublishForm`'s preview button passes `getPage().getPageReference()` when storing the preview.
• `PreviewPage`'s Back button calls `setResponsePage(ref.getPage())` to return to the live page instance; falls back to `RestartResponseException(PublishPage.class, pageParams)` if the reference has expired from the page store.
• `PublishPage` / `GenPublishPage` drop the `formobj` redirect and session-cache lookup — just `new PublishForm(...)`.
• `NanodashSession` drops `formMap`, `MAX_FORMS`, the LRU eviction + logger, and the `setForm`/`hasForm`/`getForm`/`removeForm` helpers (plus the `removeForm` call on successful publish).

Why

• The page store (capped at 100 MB/session) already preserves stateful pages across requests. The formobj layer was a second cache in front of it, solving the same problem less reliably — it never had more guarantees than the page store, only a smaller LRU (20 entries) that caused premature evictions.
• `PageReference` is the idiomatic Wicket API for "go back to the page I came from".
• Net: -103 / +19 lines.

Test plan

• Fill a template form, click "Preview", click "Back" — form values are still populated.
• Fill a template form, click "Preview", click "Publish" — confirm page renders, no errors.
• Fill a template form, fail validation, then preview still works once errors are fixed (sanity check that Placeholder values swapped when removing statement after form validation error #445's fix isn't disturbed).
• Same three checks via the connector flow (`GenPublishPage` → `PreviewPage`).
• Open the browser's network tab and confirm `/publish` URLs no longer carry `&formobj=...`.

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