[hydropix]

Summary

Fixes the Styles tab regressions reported against 1.5.15.

• Non-ASCII names (风格无法编辑 #296). slugify_preset_name kept only [A-Za-z0-9_-], so the localized default name of a new style (新建空白风格 in zh-CN, likewise ja/ko) slugified to empty and every save failed with "Preset name cannot be empty". Letters and digits of any script are now kept.
• Hand-renamed files disappeared (Regression and inconvenience regarding the styles section (TBL 1.5.15) #293.1). SAFE_FILENAME_RE rejected spaces, parentheses and accented/CJK letters, so Web Novel (v2).txt was silently skipped by the listing. Those are now accepted. Path separators, Windows-forbidden characters and leading dots or spaces stay rejected, and resolve_inside still guards containment.
• No way to name a preset, and a second blank style failed (Regression and inconvenience regarding the styles section (TBL 1.5.15) #293.2, Regression and inconvenience regarding the styles section (TBL 1.5.15) #293.3). The editor header now has an editable name field. It names new presets, and on an existing preset PUT accepts name to rename it (the assignment follows if the preset was assigned). The default-name check now compares slugs instead of display names against file stems, which is what made a second "New blank style" hit a 409. Taking an existing name asks for confirmation on create and returns 409 on rename, so a preset is never overwritten silently.
• Legacy .txt presets could not be saved or deleted. Both paths were YAML-only and returned 400. A .txt is now written back in place while it keeps a single text for both phases, and converted to .yaml (the .txt removed) once the phases diverge or metadata is added.
• Textareas could not be enlarged (Regression and inconvenience regarding the styles section (TBL 1.5.15) #293.4). .form-control's fixed single-line geometry pinned all 7 multi-line fields of the app to 44px. textarea.form-control now sizes from rows and resizes vertically.
• Export of a non-ASCII preset name now sends an RFC 5987 filename* header.

The feature requests in #293 (txt-only workflow, in-app rename/duplicate) are partly covered: .txt presets keep working and are now editable, rename is new, duplicate already existed.

Test plan

• New endpoint tests: CJK create, hand-named listing, unchanged-name PUT, rename, rename conflict leaves both files untouched, .txt saved in place, .txt converted to .yaml, .txt delete, non-ASCII export header.
• Updated is_safe_filename / slugify_preset_name unit tests.
• Full suite: 2604 passed. The 5 tests/characterization/test_progress_baseline.py failures are the goldens that are already stale on main.
• Browser, zh-CN UI: created "新建空白风格", second default became "新建空白风格 2", rename conflict showed an error and changed nothing, rename to "武侠 风格" worked, Web Novel (v2).txt listed and saved in place, name placeholder updated on switching to FR without reload.

Fixes #296
Refs #293

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