Add click-to-cast target selection and the original casting flow

[RahulDNair]

What this adds

The complete skill-casting flow of the original client, reimplemented clean-room from observed behavior (no code was taken from other clients — behavior was studied via roBrowserLegacy's semantics and verified against rAthena's server implementation):

• Click-to-cast target selection: clicking a skill in the hotbar (or pressing its function key) switches to a target selection mouse mode using the original client's target cursor from cursors.act. The next left click casts at the entity or tile under the cursor; right click or escape cancels. Self-cast skills fire immediately, passives do nothing.
• Function key hotbar: all ten hotbar slots are now mapped to F1–F10 (previously J/L/U covered three slots with instant casting). Keys and clicks share one code path.
• Cast level adjustment: while selecting a target, the mouse wheel adjusts the level the skill will be cast at (clamped to [1, learned level]), displayed next to the cursor — the classic "level 7 bolt" trade-off between cast speed and damage.
• Walk into cast range: casting at an out-of-range target walks the player toward it and sends the cast request immediately. rAthena remembers skill requests received while walking (ud->stepaction) and executes them per walk step against the target's current position, so the cast follows moving targets and fires as early as possible. Ground skills included. This deliberately relies on the server's step-action machinery rather than a client-side buffer (which would double-cast).
• Ground-skill cell indicator: the hovered tile is highlighted while targeting a ground skill.

Fixes found along the way

• Skills at maximum level could not be dragged to the hotbar — the pickup handler rejected skills whose upgradable flag was false, which is every maxed skill. It now only rejects unlearned skills.
• The target cursor rendered the wrong sprite action — the cursor direction workaround maps action indices correctly only for the states used so far (the index wraps modulo the action count); the target cursor is action 10 and rendered action 3. Even-numbered states need direction zero.
• Skill damage was invisible — ZC_NOTIFY_SKILL2 (0x01DE) was registered as a noop, so skill hits showed no damage numbers and no hit reaction. It now emits the same damage event as auto attacks.

Tested

Live against a local rAthena (PACKETVER 20220406): bolt skills at both level extremes (visibly different cast time and damage), targeting cancel via escape/right-click, casting at moving targets while walking into range, ground skill placement, drag-and-drop and slot swapping unchanged.

Known follow-ups (out of scope here)

• The original client gates level adjustment per skill (bSeperateLv); this PR allows it on any targeted skill, and the server clamps regardless.
• The original client shows the skill name and description overlay during target selection.
• Support-priority entity picking during friendly-skill targeting.

This builds on the same play-testing effort as #419 but is independent of it.

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